The present invention relates generally to plastic containers. More particularly, but not by way of limitation, this invention relates to an improved method and apparatus for making containers, such as bottles and cans, having improved gas transmission barrier characteristics.
In the food and beverage industry the trend is to move away from packaging perishable products in glass and metal containers and to substitute thermoplastic polymers for the container material. One of the most successful polymers for beverage containers to package beer, wine, and soft drinks has been polyethylene terephthalate (PET). One of the largest markets for PET containers has been in the two-liter carbonated drink field. Another area where PET is expected to be used extensively is in packaging beer and food. In either case, one of the most critical characteristics of the polyester package is the prevention of gas permeation through the wall of the container.
With carbonated soft drinks, the problem with gas permeation is the loss of carbonation (C0.sub.2 gas) from the drink through the wall of the bottle or can. Compared to the small, densely-packed metal and glass molecules, polymer molecules are relatively large and form a permeable wall. Even the best polymer known at this time for gas barrier properties, ethylene vinyl alcohol (EVOH), has poor barrier ability when compared to the inorganics such as metals and glass.
On the other hand, beer and food containers preferably should present a good vapor barrier against the ingress of oxygen (O.sub.2) into the container because of the accelerated spoilation of the food products caused by the presence of oxygen therein.
There have been several different methods developed in an attempt to increase the "shelf-life" of plastic containers. One of the most common methods involves creating a multi-layered container having a thin barrier layer of a material such as EVOH or polyvinylidene chloride (PVdC) buried between two or more layers of a container polymer such as PET, polypropylene, polystyrene, or PVC. This multi-layer container is difficult and expensive to manufacture since the barrier layers are either expensive (EVOH) or corrosive (PVdC). Also the process for forming a multi-layered material and making a container from it may be much more complex than single-layer processes.
Another method of creating a barriered polymer container is the process known as "dip-coating". In this process a polymer bottle made of a material such as PET, is first formed into its final shape and then the additional step of dipping the container into a coating solution is performed. This solution may be of a barrier material such as PVdC. This process, in addition to adding another expensive step to the container manufacture, also introduces a material to the container that prevents easy recycling. Because of the nature of PVdC, the coating must be removed by solvents before the polymer container can undergo normal recycling. In light of the trend toward compulsive container return laws in various states and a probable federal deposit/return law, all future container designs must be quickly and easily recyclable. Dip-coated bottles do not lend themselves to easy recycling.
U.S. Pat. No. 4,478,874 issued Oct. 23, 1984 to Granville J. Hahn describes an additional process for improving the gas barrier characteristics of thermoplastic containers. The process described in the patent involves ion-plating the container with a thin, flexible layer of an inorganic oxide.
The present invention overcomes the deficiencies of the barrier-layer containers and the dip-coated containers by providing a plastic container which provides excellent barrier characteristics, is cheaply and easily treated, and can be completely recycled by conventional recycling techniques without need for removal of dip-coated layers. This is achieved by impregnating the interior surface of a normal polymer container with an inorganic material such as a metallic oxide. The impregnation is done by gasless ion plating to provide an ultra-thin flexible coating of the inorganic material on the plastic substrate.